The invention relates to liquid fittings for corrosive liquids under pressure in general and more particularly to an improved fitting such as a liquid flow meter for measuring corrosive liquids under pressure.
Liquid flow meters for measuring liquids under pressure must be designed so that their parts which are important to the measuring process, i.e., in particular, the measuring chamber which is machined to close tolerances, are either not deformed or only slightly deformed under the pressure of the liquid. Otherwise the functioning, or at the every least, the measuring accuracy of the equipment would be adversely affected to a large extent.
To satisfy these requirements in known commercially available liquid flow meters of the type having a pressure resistant outer housing in which a measuring chamber housing with a movable measuring element is arranged, and a space filled with the liquid to be measured between the outer and the measuring chamber housing, the space between the outer and the measuring chamber housing is filled with the liquid to be measured under pressure. As a result, the pressure outside and inside the measuring chamber housing is practically the same and no mechanical pressure is then exerted on the measuring chamber housing, so that there is no adverse effect on the measuring accuracy. If chemically corrosive media are to be measured with the known liquid flow meters, which is quite often the case, then the outer housing must also be made of high quality material because of the required corrosion resistance. Since the outer housing must be able to withstand the pressure of the liquid to be measured, it must be made appropriately strong, which requires a relatively large amount of valuable material.
To reduce the required amount of valuable material needed for the outer housing, there have been attempts to provide the outer housing with a corrosion resistant plastic lining. However, this presents great difficulties, among other things, with respect to dimensional compatibility, since accurate fitting of the plastic lining into the outer housing is necessary, so that the pressure prevailing within the outer housing is transmitted to the outer housing without damaging the plastic lining. The range of application for using a plastic lining, furthermore, is relatively narrow, for instance, because of the temperatures permissible for plastic.
In another attempt to solve the problem, a liquid flow meter which can be manufactured inexpensively even in a design for measuring corrosive liquids under pressure, has been proposed. In such device the space between the measuring chamber housing and the outer housing is tightly separated from the liquid to be measured and is kept in pressure equalizing connection with the liquid to be measured by means of at least one element which is movable by pressure.